1. Field of the Invention
The present invention relates to a rewritable photochromic optical disc having a photochromic recording layer.
2. Description of Background Information
The EDRAW (Erasable Direct Read After Writing) optical disc is known as one of the rewritable memories capable of recording, reading, and erasing. Among EDRAW optical discs there is a rewritable photochromic optical disc provided with a thin film of organic photochromic material. It is capable of achieving a higher recording density than the magneto-optical disc which is provided with a thin film of rare earth-transition metal alloy having a magneto-optical effect.
The organic photochromic materials are compounds, which react with light to repeatedly change from a chromatic state to an achromatic state or vice versa. As shown in light-absorption spectral distribution curves of FIG. 1 by way of example, the materials have the following characteristics. When light of a wavelength B is absorbed by the organic photochromic material, it changes from a stable state X to a quasi-stable state Y in which it is rendered chromatic. When it absorbs light of a wavelength A or heat under the quasi-stable state Y, it returns to the original material again and is rendered achromatic. By utilizing the above described phenomenon, i.e., photochromism, a rewritable optical disc is produced, to and from which information is recorded, reproduced, or erased by a laser beam. As such organic photochromic materials, there are for example, those materials such as thioindigo which make use of the isomerization reaction with light, spiropiranes which utilize the ring opening/closure reaction and a redox reaction or the like, and flugido, etc.
An erasable optical disk, which, for example, makes use of the isomerization reaction with light and heat which is reversible as in the organic photochromic materials, is produced by the following process. As shown in FIG. 2 a recording layer 2 including the organic photochromic material is formed as a thin film on a transparent substrate 1 and then an aluminum-made reflecting layer 3 is formed on the recording layer 2 in the form of a thin-film. On the main surface of the substrate 1, grooves used for tracking, prepits for controlling the writing and reading of data, and preaddresses 5 are formed in advance by the process of stamping or the like. A laser beam 6 is irradiated on the photochromic material through the substrate 1 of the disk.
Recording of information on the optical disk is performed by applying light and heat energy to the recording layer 2 by means of a writing laser beam formed as a spot through an optical system of the pickup including an objective lens 4, generating the isomerization reaction positively, thereby forming a train of spots which assume changes in the tone or refractive characteristics. The reading of information from the optical disk is performed by irradiating a reading laser beam having another wavelength so as to read out the train of the spots which have been recorded.
The EDRAW disk can thus be obtained which is capable of recording information at a relatively high density.
With the EDRAW disk, further increase in recording density requires that the laser beam spot and the recording track are made much smaller. However, what is to be achieved to this end is limited in both cases.